Mike I have a Stihl EDT6 I picked up a few weeks back and love it, I am assuming that the EDT 303 is the larger model which you hold in your hand and clip on to the plug or electronic system. According to my dealer the 303 does the same things as EDT6 + it can take some readings to analyse electronic modules and electronic system problems etc. He has one but hardly ever use it so he claims, he tells me they mostly use the tiny EDT units and I did see 2 of them, one attached to each bench in the shop.
He refused to sell me the larger unit claiming I would not find as handy, practical and as usable.
I love the tiny EDT6 unit, you hold it within 2 - 3 inches of the plug without the need of a wire and it will read the rpm's, once you pull it away it displays the highest rpm's read during that session. A must have to adjust chainsaws properly or any 2 or 4 cycle engine for that matter.
Perhaps StihlTech here can comment further on these.